Aircraft are typically optimized in design for an intended purpose. In particular, commercial airliners, such as the Boeing 747, are specifically designed for carrying either passengers or cargo in their primary mode. Also, the passenger and cargo models utilize a large commonality of structural features to make manufacture economical. Specifically, the freighter version has seat track installations that are interchangeable with those of the passenger version, at manufacture, even though a freighter is not designed primarily for seat installation and passenger carriage. However, these tracks are useful for accommodating the attachment of freighter cargo-handling components. Furthermore, there are available passenger/cargo and convertible models of 747 aircraft.
Typically, the load resulting from passengers or palletized or packaged cargo results in a rather distributed load and is carried by a modest floor structure. The floor structure consists, in part, of transverse floor beams that are connected to a monocoque skin of the fuselage through circumferential frames. Also included in the floor structure and lying on top of the transverse floor beams are longitudinal roller beams that take the cargo loading from the pallet through the rollers. Although the strength of a pallet does distribute concentrated loads, engineering design does not credit standard military pallets with this capability because their structural integrity is dependent upon their treatment or mistreatment in the field. Therefore the concentrated load of a heavy vehicle wheel is treated differently than that of packaged cargo resting on a pallet.
The freighter floor structure typically is composed of a plurality of shallow longitudinal roller tracks (roller beams with rollers installed) that are attached to the transverse floor beams and provide a structure upon which cargo pallets can slide for positioning in the aircraft. These roller beams also support the loaded pallets in their restrained positions and transfer loads to the underlying floor beams. The roller tracks run on opposite sides of the aircraft centerline. Typical military cargo pallets are comprised of aluminum skin laminated to a balsa wood core and have a thickness of approximately 2 to 4 inches and an area of approximately 109 by 200 inches. Seat track structure is typically positioned to accommodate passenger seat rows, and in these locations is used on freighters for cargo tie-down and attachment of cargo-handling components. The seat tracks are attached to the transverse floor beams.
Military cargo aircraft have stronger floor systems to support the high axle loads of military vehicles which may be transported. Static axle loads of 8,000 pounds are not uncommon. Furthermore, the aircraft may typically be subject to positive and negative vertical accelerations on the order of +2.5 gravities and -1.0 gravity, respectively. A substantial number of civilian cargo aircraft are therefore capable of performing only limited military cargo service as presently designed. The carriage of heavy vehicle, concentrated axle loads is the missing capability. It would be highly advantageous to increase use flexibility by temporarily converting these civilian cargo aircraft for heavy vehicle carriage when used for military service in a time of national emergency. Therefore, a need exists for a device which can temporarily increase the floor structure load capacity of a civilian cargo aircraft during use for military duty.